Variable resistors have been produced by the millions over a period of 50 years or more. Except for special types for particular applications, the design has usually consisted of a carbon powder mixture which has been printed, sprayed, or otherwise applied to an insulating base. The base normally used has been paper with a phenolic binder, providing adequate insulating properties for the carbon resistance track. A movable wiper has been the usual means of selecting the correct point on the track corresponding to the desired resistance. This wiper can be rotated by an affixed shaft or can be slotted or shaped to accept an actuator such as a screwdriver.
The paper base used as the insulation has two major deficiencies in that it is limited as to the temperature that it can withstand and it is subject to absorption of moisture. When moisture is absorbed, the bond between the base and the resistance track or coating is broken and the resistor becomes inoperable. The paper also lacks good mechanical strength and can be easily broken. One of the primary problems thus encountered with the use of phenolic based variable resistors has been that of unreliability. Normally the portions of the variable resistor at each end of the resistance track require that a transition be made from the resistance element to a metal terminal suitable for circuit soldering and mounting. The same difficulty sometimes exists in completing the circuit from the moveable wiper to the terminal. Unfortunately a solid bond cannot be made because the insulating base cannot withstand high temperature such as would be required with soldering, for example. When the heat of soldering a wire to the terminal, or when the terminal is dipped in a solder bath, the insulating material of the base can be damaged, loosening the mechanical fastening used to press the terminal against the carbon track. Even more likely, some of the resin used during the soldering operation may flow up the terminal and get between the terminal and the carbon film or coating.
Circuit designers, in their attempt to avoid failures, have weighed the costs of much higher quality designs versus the costs of product failure. In some cases an approach using a type of variable resistor built on a ceramic base has been selected, but this usually increases cost to about twice the cost of the phenolic base type. Obviously, in the quest for lower and lower costs, this selection cannot be tolerated permanently. The subject invention eliminates this problem without substantial cost increase.
An important difference in the structure embodying the present invention concerns the use of a metal as the base. The metal can be of a wide variety, selected for specific attributes. Steel would be the most usual selection, although brass, stainless steel, or various alloys could be used. A metal base has several advantage in addition to its obvious structural strength. Its moisture absorbence is non-existent, permitting a predicted stability, even under long storage periods at various temperatures. This ensures the preservation of the resistance track in its applied form. It also provides a remarkable improvement in power handling capability. In addition to the much higher operating temperatures that can be accommodated simply because the base is metal, the heat sink capabilities are important. No hot spot can develop and heat concentrated in a particular portion continuously dissipates by conduction into the reservoir formed by the remainder of the structure.
The feature of primary importance, however, of the present invention concerns the establishing of permanent continuity between the resistance track and the solder terminal. In order to insulate the resistance element or film from the metal base an insulating surface or coating must be applied. The metal base, preferably, has a tin plated surface to provide initial protection and solderability. This insulator coating could be a lacquer, a polyimid, such as DuPont's Pyre M-L, a tin-oxide, or if aluminum were used, an anodize. Other substances could be selected. In order to achieve the continuity desired, it is necessary to mask the portion of the metal base from the insulating coating, this portion becoming the terminal. The resistance film mix is then placed over the insulating coating and continued, in its application, past the insulated portion of the base into the exposed portion. The resistance material bonds to, or can be bonded by soldering to the base metal or its plated surface, for example, at the point of the base corresponding to the terminal. This terminal can be at any of the three common potentiometer positions and would have no mechanical joint, thus ensuring reliability.